Linerboard is a heavy weight paper, preferably kraft paper, that is used for the manufacture of corrugated cartons and the like. Medium is the configured material that is placed between the walls of the linerboard to make the corrugated structure. It has long been recognized that these materials need to be strong to maintain the integrity of corrugated blanks and to protect the contents of corrugated boxes properly during use.
One of the most important properties for linerboard and medium is the crush resistance. This is the ability of the material to resist forces applied in a columnar or compressive manner, such as when cartons are stacked in warehouses. Cartons must also resist end-to-end compressive forces during shipment. The compressive strength of corrugated materials can be measured in several ways. One way is to form the material into a ring with a diameter of approximately 2 inches, then crush the ring. The ring structure reinforces the board so that a true compressive failure occurs, rather than a mere bending. This is called the ring crush test.
The art has recognized that starch can often be applied to the surface of paper to "size" the surface to provide a smooth texture to the paper for writing. It is generally known that much of the starch applied to paper with a size press remains close to the surface. Those in the art have regarded this feature as advantageous, drawing a parallel to an "I-beam" that has good stiffness because it concentrates the mass near the extremities of its dimensions. See, e.g., Paulapuro et al., "Effect of Pulp and Papermaking Variables on Linerboard Ring Crush," Pulp and Paper, pp.142-145 (1983), which is hereby incorporated by reference. However, there are limitations as to how much starch can be applied to the surface of a linerboard. It is generally known that starch can close up the surface pores of a paper and retard the penetration of corrugation adhesives. This can either prevent bonding or result in weak bonding between the corrugated medium and the facing.
Others have recognized that it may be desirable to achieve penetration of starch into low basis weight or fine paper in order to build up the internal properties of the sheet for surface sizing applications. Casey, Pulp and Paper, 2nd Ed., p.1114, Interscience Publishers, Inc., New York (1960); Lee, "Biological Methods in Paper Diagnosis," 107(6) Paper Trade J. 53-59 (1938), which references are hereby incorporated by reference. Again, however, the process as described by Casey is more concerned with the overall smoothness of the paper surface for subsequent processing and not for improving crush strength.
Still others have done work on various methods of starch deposition. These include U.S. Pat. No. 2,772,604 (Muggleton), U.S. Pat. No. 4,093,016 (Coleman), and K. P. Simons, "Leimpresse Und Starke-- Praktische Erfahrungen," 115(22) Wochenblatt Fur Papierfabrikation 1004 (1987).
The invention described in U.S. Pat. No. 4,191,610 (Prior) relates to use of sulfite waste liquor in a manufacturing process in order to improve the physical properties of the resulting paper. In using the sulfite waste liquor in a modified form, Prior allows for the utilization of mixed waste material as a starting material for producing containers such as corrugated boxes. The key goal of the Prior invention is the ability to make use of, and thereby dispose of, sulfite waste material (which has been considered a major pollution problem in the industry). Only as an addition to the sulfite waste liquor is the use of starch mentioned or discussed (see col. 7, lines 34-52.)
There is no implication in the Prior patent that improved starch application and the placement of significant amounts of the applied starch in the intermediate zone of the product can be accomplished in the absence of sulfite waste liquor. Prior teaches away from the accomplishment of such starch distribution, at least in the absence of sulfite waste liquor, by implying in its background discussion that starch would build up on the surface of the product and cause excessive brittleness (col. 3, lines 26-29).
Those of ordinary skill in this art field at the time of the present invention believed that using a heavier weight medium would, if anything, lead to a thicker medium that would be more resistent to effective starch penetration into the middle of the medium. At the same time, use of a higher starch solid composition would result in a thicker starch solution that would not penetrate as well as a lower starch solids solution. See N. O. Bergh, D. Glittenberg, H. Weinbach, "Oberflachenbehandlung von Papier mit Leimpresse und Filmpresse," JAHRGANG HEFT 42:10A:V-40-V-47 (1988). If the starch solid composition and/or the weight of the medium were increased, the state of the art would have predicted that there would be much less penetration of the starch. Thus, the ordinarily skilled artisan was led away from using higher starch solids compositions.
Accordingly, those skilled in the art have not demonstrated an appropriate teaching for significantly increasing the crush strength of corrugated materials by chemical means, such as starch.